Measuring Capacitance

1. Aim

To measure capacitance connected between IN1 and GND, and study how geometry affects capacitance.

2. Apparatus / Components Required

3. Theory & Principle

Capacitance is defined as: \(C=\frac{Q}{V}\)

SEElab measures small capacitances (pF and nF ranges) by charging with a constant current source and measuring the voltage rise produced. Q is the product of C and V, and Q can be calculated as a product of this constant current and the time spent charging. For a parallel-plate capacitor: \(C\propto\frac{A}{d}\) More explicitly (same dielectric): \(C = \epsilon \frac{A}{d}\) where A is plate overlap area and d is separation.

For larger values, it charges the capacitor via a built-in 10K resistor whilst simultaneously capturing the capacitor voltage using the oscilloscope. All this magic happens internally, and the capacitance value is extracted after fitting the charging curve to an appropriate function to extract the time constant.

Mobile App

Mobile App

Desktop App

Photo with ExpEYES17

4. Circuit Diagram / Setup

  1. Connect capacitor between IN1 and GND.
  2. Open the “Measure Capacitance” tool and trigger measurement.
  3. Repeat with different capacitors.

5. Procedure

  1. Measure known capacitors and record values.
  2. Build a parallel-plate capacitor using foil-paper-foil.
  3. Measure capacitance for full overlap.
  4. Reduce overlap area gradually and re-measure.
  5. Compare trends in measured values.

6. Observation Table

Capacitor Type Expected Value Measured Value Remarks
Ceramic (nominal)      
Electrolytic (nominal)      
Homemade plate capacitor (full area)      
Homemade plate capacitor (reduced area)      

7. Advanced: Area Dependence Check

For same dielectric and separation: \(\frac{C_2}{C_1} \approx \frac{A_2}{A_1}\)

Example: if overlap area is reduced to half, measured capacitance should be approximately half.

8. Error Analysis

9. Precautions

  1. Do not touch capacitor plates during measurement.
  2. Keep leads short for pF-range measurements.
  3. Ensure capacitor is discharged before reconnecting.
  4. Use firm clips and avoid movement while reading.

10. Troubleshooting

Symptom Possible Cause Corrective Action
Reading too high Stray/body capacitance Keep hands away, shorten leads
Reading unstable Loose connection Tighten clips
No reading Wrong terminal Ensure capacitor is between IN1 and GND

11. Viva-Voce Questions

Q1. Why should you not touch capacitor plates while measuring?

Ans: Touch introduces leakage and extra capacitance, changing measured value.

Q2. How does capacitance change with plate overlap area?

Ans: Capacitance is directly proportional to overlap area.

Q3. What happens when plate separation increases?

Ans: Capacitance decreases because $C$ is inversely proportional to separation.